1. Field of the Invention
The invention relates to a device for displaying television pictures comprising a display tube, the neck of which contains an electron gun system for emitting at least one electron beam towards a display screen. A deflection unit is mounted coaxially about the display tube, which deflection unit comprises a line deflection coil system which when energized deflects the electron beam(s) in a first direction and a field deflection coil system which when energised deflects the electron beam(s) in a second direction transverse to the first direction, the said field deflection coil system comprising two field deflection coils each of the saddle type situated diametrically with respect to each other, each deflection coil comprising a plurality of conductors forming first and second lateral winding packets extending in the longitudinal direction of the deflection unit which substantially coincides with the direction of the longitudinal axis of the display tube. An arcuate front end section and an arcuate rear end section together define a window. The invention also relates to a deflection unit for use with such a display device.
In monochrome display tubes the electron gun system is designed to generate one electron beam, whereas in colour display tubes of the inline type the electron gun system is designed to generate three coplanar electron beams which converge on the display screen.
2. Description of the Prior Art
The deflection unit placed around the display tube and destined to deflect the electron beams is used to deflect the electron beams in one or in the opposite direction away from their normal undeflected straight path, so that the beams impinge on selected points of the display screen to provide visual indications thereon. By suitably varying the magnetic deflection fields, the electron beams can be moved upwards or downwards and to the left or to the right over the (vertically arranged) display screen. By simultaneously varying the intensity of the beams, a visual presentation of information or a picture can be formed on the display screen. The deflection unit connected around the neck part of the display tube comprises two deflection coil systems to be able to deflect the electron beams in the two directions which are transverse to each other. Each system comprises two coils which are placed on opposite sides of the tube neck, the systems being located 90.degree. around the tube neck with respect to each other. Upon energization of the two deflection coil systems produce orthogonal deflection fields.
Essentially the fields are at right angles to the path of the undeflected electron beams. A conical core of magnetizable material which, when both deflection coil systems are of the saddle type, may be situated tightly around the deflection coil systems, is mostly used to concentrate the deflection fields and to increase the flux density in the region where deflection is performed.
In order to satisfy certain requirements as regards the picture quality, the (dynamic) magnetic deflection fields should often be strongly modulated. For example, the ever more stringent requirements as regards convergence in three-in-line colour television systems necessitate, in addition to a strongly negative magnetic six-pole component in the central region of the field deflection magnetic field, a strong positive magnetic six-pole component on the gun side of the deflection field. The strong positive six-pole component is necessary for field coma correction. (The effect of a positive six-pole component on the dipole deflection magnetic field is a pincushion-shaped field variation). For a self-converging in-line colour system having green as central beam and red and blue as outer beams, coma is to be understood to mean a vertical shift of red and blue with respect to green. If no measures are taken to correct coma, red and blue will be deflected more strongly than green. With a pincushion-shaped deflection magnetic field on the gun side, red and blue experience a weaker deflection field than green. Red and blue will hence be deflected less strongly.
Display devices of the type described in the opening paragraph comprise field deflection coils of the saddle type. They are self-supporting coils which comprise a number of conductors which are wound so as to produce first and second lateral winding packets, an arcuate front end section and an arcuate rear end section which together define a window. In such coils, the rear end sections (at the end adjacent the tube's gun) may be turned up with respect to the profile of the display tube (the original type of saddle coil) or lie parallel (in this type of saddle coil the rear end section follows the tube profile, as it were).
A pincushion-shaped magnetic field is generated when the window apertures of the two saddle coils of a system of deflection coils are large. A barrel-shaped field is generated when the window apertures are small. For a self-converging system the field deflection coil system about its central or middle area must have a barrel-shaped distribution (the individual saddle field deflection coils must thus have a small window aperture), at the gun end a pincushion-shaped distribution (large window aperture), and at the screen end a homogeneous or more or less pincushion-shaped distribution, dependent on how much east-west raster distortion can be tolerated. Similar field distributions are also of importance for monochrome display tube/deflection unit systems which are to have a high resolving power.
It has so far not proved possible to manufacture, by means of the current winding methods, saddle coils having such a strongly varying width of the window aperture as is desired for the said applications. On the other hand, several compromise solutions are known to reduce the problem. For example, a reduced window than is in fact necessary will suffice at the gun end by locally making the field deflection magnetic field pincushion-shaped by means of segments of soft-magnetic metallic material placed within the field deflection coil system, transversely with respect to the direction of the field deflection magnetic field. The use of such segments, however, is detrimental from the energy consumption point of view because they partly screen the generated field. Moreover, their effect is restricted.
It is the object of the invention to provide a coil design for a display system of the kind described in the opening paragraph which leads to the generation of a field deflection field having the field shape desired with respect to self-convergence and east-west distortion, having means on the gun side which intensify the pincushion shape and which do not adversely influence the sensitivity of the field deflection system.